The present invention relates to a method and apparatus for increasing the temperature and for cold start-up of an electrochemical fuel cell using reactant starvation at an electrode. More particularly, the method comprises fuel starving at least a portion of the anode of an operational fuel cell or oxidant starving at least a portion of the cathode of an operational fuel cell or both to increase the temperature. The method may be used, for example, during start-up or during operation of the fuel cell when the temperature of the fuel cell is below the preferred operating temperature range. Thus, the method and apparatus may be used to heat the fuel cell and to prevent poisoning of electrode catalysts while allowing for some generation of power by the fuel cell during start-up.
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to produce electric power and reaction products. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) comprising a solid polymer electrolyte or ion-exchange membrane disposed between two porous electrically conductive electrode layers. The anode and cathode each comprise electrocatalyst, which is typically disposed at the membrane/electrode layer interface, to induce the desired electrochemical reaction.
At the anode, the fuel moves through the porous anode layer and is oxidized at the electrocatalyst to produce protons and electrons. The protons migrate through the ion exchange membrane towards the cathode. On the other side of the membrane, the oxidant moves through the porous cathode and reacts with the protons at the cathode electrocatalyst. The electrons travel from the anode to the cathode through an external circuit, producing an electrical current.
Electrochemical fuel cells can operate using various reactants. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be substantially pure oxygen or a dilute stream such as air containing oxygen.
In some applications, fuel cell systems may operate almost continuously (e.g., certain stationary power applications). However, in other applications, fuel cell systems may be subjected to frequent start and stop cycles and to prolonged storage periods in between (e.g., portable or traction power applications). Further, in colder climates, such fuel cell systems may frequently be subjected to temperatures below freezing. Such systems therefore must tolerate exposure to sub-zero temperatures without degradation. Additionally, the power output capability from fuel cells is typically very limited at temperatures well below the normal operating temperature. Thus, it is also desirable to be able to start up such systems and bring them up to normal operating temperature in a timely, energy efficient manner, and to maintain the temperature within a desirable range during operation.
A conventional approach for starting up a fuel cell is to employ an external power source (e.g., storage battery) or a heater to heat the fuel cell up to a temperature at which fuel cell operation is commenced. However, this requires additional equipment just for start-up purposes and generally requires a net input of energy during start-up. Problems encountered below freezing may simply be avoided by not allowing the fuel cell temperature to go that low. In many applications however, this is not practical. Another approach for low temperature start-up involves operating the fuel cell during start-up and using some of the power and heat generated within to bring the fuel cell up to normal operating temperature. For instance, U.S. Pat. No. 5,798,186 discloses a method for starting up a solid polymer fuel cell stack involving supplying power from the stack to an external load, and then increasing the power drawn and, optionally, the flow rate of the reactant streams while the stack warms up. Another starting method is disclosed in Japanese Patent Publication (Kokai) No. 07-302607, in which the contact resistance between components in the main body of a fuel cell is increased by reducing the pressure applied to the main body of the fuel cell. Internal energy losses are increased and thus the fuel cell temperature can be increased without using an external power source or heater.
A further complication during start-up of fuel cell systems relates to the possible presence of impurities in the reactant streams, particularly the fuel stream. The fuel stream may contain impurities known as electrocatalyst xe2x80x9cpoisonsxe2x80x9d which may adsorb or deposit on the anode electrocatalyst and inhibit the desired electrochemical reaction on the anode. The presence of poisons on the electrocatalyst thus results in reduced fuel cell performance. In the absence of countermeasures, the adsorption or deposition of electrocatalyst poisons may be cumulative, so even minute concentrations of poisons in a fuel stream, may for instance, over time, result in a degree of electrocatalyst poisoning which is detrimental to fuel cell performance. Further, the poisons may adsorb or bind more strongly at lower temperatures thereby aggravating the adverse effect on performance at lower temperature.
Reformate streams derived from hydrocarbons or oxygenated hydrocarbons typically contain a high concentration of hydrogen fuel, but typically also contain electrocatalyst poisons such as carbon monoxide. To reduce the effects of anode electrocatalyst poisoning, it is known to pre-treat the fuel supply stream prior to directing it to the fuel cell. For example, pre-treatment methods may employ catalytic or other methods to convert carbon monoxide to carbon dioxide. However, known pre-treatment methods for reformate streams cannot efficiently remove 100% of the carbon monoxide. Even trace amounts less than 10 ppm can eventually result in electrocatalyst poisoning, particularly at low temperatures. Further, during start-up of a reformate-supplied fuel cell system, the reformer and other related apparatus for pre-treatment must themselves also be started up and brought up to a desirable normal operating temperature. Typically, during start-up, the reformer and pre-treatment apparatus are not as effective in providing fuel with a low level of impurity. Thus, the level of poisons in the reformate is typically higher during start-up than it is at normal operating temperature.
It may be possible to remove electrocatalyst poisons by purging the affected electrode with an inert gas such as nitrogen or with a xe2x80x9ccleanxe2x80x9d reactant stream containing substantially no poisons. Where the adsorption of the poison is reversible, an equilibrium process results in some rejuvenation of the electrocatalyst. However, this approach is not as effective against strongly bound adsorbed poisons and can be very slow. Due to the additional difficulties posed by electrocatalyst poisons during system start-up, often reformate is not supplied to a fuel cell system until both the reformer and fuel cell systems are close to the preferred normal operating temperature.
An improved method of heating a solid polymer electrolyte fuel cell employs reactant starvation over at least a portion of at least one of the fuel cell electrodes, thereby increasing the overvoltage at that portion. Additional heat generation takes place as a result of the starvation. The method is particularly useful for starting purposes in that it provides for faster start-up. The method allows for the provision of some electrical power output from the fuel cell during the starting period. The method is useful for heating or starting up fuel cells supplied with reactant streams that are essentially free of electrocatalyst poisons (e.g., a fuel reactant stream of pure hydrogen). However, the reactant starvation method can also serve to remove electrocatalyst poisons that are introduced in a reactant stream. Thus, the method is particularly useful for starting up fuel cells supplied with a reactant stream comprising an electrocatalyst poison.
For starting purposes, the method involves starting the fuel cell from a starting temperature below its normal operating temperature and the temperature of the fuel cell rises to the normal operating temperature over a starting period. The method comprises supplying an oxidant reactant stream to the cathode electrode of the fuel cell, supplying a fuel reactant stream to the anode electrode of the fuel cell, and reactant starving at least a portion of one of the electrodes during the starting period, thereby increasing the overvoltage of the portion of one of the electrodes and generating additional heat. The reactant starvation may be stopped before the normal operating temperature is reached once the fuel cell temperature has reached a predetermined threshold temperature.
For temperature regulation purposes generally during operation, the method involves operating the fuel cell while supplying an oxidant reactant stream to the cathode electrode of the fuel cell and a fuel reactant stream to the anode electrode of the fuel cell method. A temperature parameter is monitored that is indicative of the operating temperature of the fuel cell. When the temperature parameter is below a predetermined threshold value, at least a portion of one of said electrodes is starved of reactant thereby increasing the overvoltage of the portion of one of the electrodes and generating additional heat. The method may be used to effect a faster temperature correction when the fuel cell temperature falls below the threshold value during operation.
Reactant starvation involves a reduction in the reactant stoichiometry. As used herein, stoichiometry is defined as the ratio, at any given instant, of the rate at which reactant is supplied to the fuel cell divided by the rate at which the reactant is consumed in the electrochemical reactions in the fuel cell. A reactant starvation condition exists whenever the reactant stoichiometry is less than 1, that is whenever less reactant is being supplied to the fuel cell than is being consumed within the fuel cell at any given instant. Such a situation is temporary since the fuel cell cannot consume reactant faster than it is supplied indefinitely. If the rate at which reactant is supplied remains constant during a starvation, the rate at which reactant is consumed will fall until it eventually matches the rate supplied, i.e., the stoichiometry eventually increases to 1.
Reactant starvation may be accomplished by interrupting the supply of one of the reactant streams to the fuel cell electrodes, thereby reducing the rate at which reactant is supplied and hence the stoichiometry. A single, optionally prolonged, interruption may be employed or an intermittent series of interruptions may be employed. Intermittent interruptions may be regular or irregular. Alternatively, reactant starvation may be accomplished by connecting a transient electrical load to draw electrical power from the fuel cell. Again, the transient electrical load may be connected once or intermittently. To effect a starvation via this method, the rates of supply of the reactants to the fuel cell electrodes are not increased to match the increased electrical demand over conventional levels in response to the connection of the transient load. Thus, this method increases the rate at which reactant is consumed and hence decreases the stoichiometry.
Apparatus suitable for heating or starting a solid polymer electrolyte fuel cell comprises an oxidant supply system for directing an oxidant reactant stream to a cathode electrode of the fuel cell, a fuel supply system for directing a fuel reactant stream to an anode electrode of the fuel cell, a temperature sensor for detecting the temperature of the fuel cell, and a control system for starving at least one of the electrodes responsive to the output from the temperature sensor. The control system may comprise apparatus for intermittently interrupting the supply of one of the reactant streams to the fuel cell electrodes, or alternatively it may comprise apparatus for connecting a transient electrical load to draw electrical power from the fuel cell. Other apparatus for achieving reactant starvation is disclosed in the aforementioned U.S. patent applications Ser. No. 08/998,133 filed Dec. 23, 1997 entitled xe2x80x9cMethod and Apparatus for Operating an Electrochemical Fuel Cell With Periodic Fuel Starvation At The Anodexe2x80x9d and U.S. patent application Ser. No. 09/344,763, filed Jun. 25, 1999, entitled xe2x80x9cMethod and Apparatus for Operating an Electrochemical Fuel Cell With Periodic Reactant Starvationxe2x80x9d.
An improved start-up may be obtained by starving at least a portion of either or both electrodes. Where electrocatalyst poisoning is also an issue, however, it is preferable to at least reactant starve the poisoned electrode (e.g., to starve the anode of a solid polymer fuel cell when supplied with a carbon monoxide containing reformate stream).
Where the fuel cell is one of a plurality of fuel cells, for example, arranged in a fuel cell stack, the method may preferably avoid the simultaneous starvation of each electrode of the plurality of fuel cells. This reduces the fluctuation in electrical power output from the stack. In fuel cell stacks, it is generally preferred to avoid voltage reversal in any of the cells. Nonetheless, it appears that a fuel cell may degrade less quickly as a result of a voltage reversal condition when it is at temperatures well below the normal operating temperature. Thus, the reactant starving method may cause a voltage reversal to occur in at least one, but preferably not simultaneously in all, of the plurality of fuel cells. Preferably, however, starvation is limited so that the voltage reversal is not prolonged.
The method is suitable for starting up a solid polymer electrolyte fuel cell and is particularly advantageous for starting up such cells from temperatures below the freezing point of water or 0xc2x0 C. In solid polymer fuel cell systems supplied with a reformate fuel stream, the method provides for a shorter starting period while also reducing the effect of carbon monoxide, methanol, or other impurities on the anode electrocatalyst. The amount of water produced during starvation is also reduced, which may be advantageous in preventing blockages due to ice formation at temperatures below 0xc2x0 C.